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Ask Dave the Car Guy: Nitrogen vs. air in tires

By Dave Herman

After reading my last column and discussing tires in class, a student asked why some of the newer cars have green caps on the tire stems.

A green stem cap means the tire is filled with nitrogen.

The idea behind nitrogen is multi-faceted, as the nitrogen molecule is larger than the oxygen molecule (even though oxygen you breath is 78 percent nitrogen) so that it will not permeate the porous side wall of the tire as easily, and in theory, will result in less pressure loss.

So let's check the numbers and we will find over time a tire filled with air loses about 1.3 pounds per square inch more than tires filled with pure nitrogen. The theory is sound.

The claim is you will improve your gas mileage by retaining proper pressure longer; or like me, you can be "proactive" and check the air in your tires every month or so while gassing up. Even if you have a car with TPMS - yeah, it does sound like a disease - Tire Pressure Monitoring Systems monitor the tire pressure, but only within 25 percent of the recommended pressure. So your tire could be 1 to 8 pounds low before the warning light comes on! You still need to be "proactive" even with this technology.

Is nitrogen economical? New car dealers charge anywhere from $65 to $170 for this added option. The average tire fill up with nitrogen is $6 per tire; you could spend $48 or more a year on tire pressure service. Not everyone has nitrogen when you need it, so you probably will have to mix air in your tires and contaminate the nitrogen. As we see, nitrogen can be an inconvenient substitute for air. Air is a readily available resource and is free, or at most 75 cents to top off four tires.

Why do road race cars use nitrogen in their tires? Because as little as half a pound difference in tire pressure can cause significant changes in the way the car handles on the track. This could mean winning second place, so the cost of nitrogen-filled tires is negligible when you factor the winnings.

The problem with using air in a racing tire is it contains moisture due to humidity, water from the air compressor, and other factors. When the race car is on the track, the tire temperature increases due to weather, track surface temperature and friction within the tire. Also, if the track has more left turns, the right side tires can run hotter then the left.

The temperature changes will vary over the course of a race. This causes the moisture in the tire to heat up and "gas," causing an increase in tire pressure and making the race car less predictable, affecting braking, cornering, stability as well as the the driver's mental state.

We remember from our previous column on economics that it is about choices and tradeoffs. Nitrogen is a "dry" gas, hence no moisture to cause erratic pressure changes vs. air, a free commodity but less reliable. So you decide. Does it make sense for street application? Is it convenient? Is it cost effective? Should it be made standard equipment, as air bags? Is it fair to all concerned?

Before his retirement, Dave Herman ran one of the largest independent German service centers in Northern Virginia. He has built, raced and taught racing to Porsche/BMW drivers. He lives in Shenandoah County, where he has opened a driving school. Email your comments and questions about anything automotive to info@nvdaily.com. Read more of his columns at www.nvdaily.com/lifestyle/guest-columns


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